Optical network communication system with optical line terminal transceiver and method of operation thereof

ABSTRACT

A method of operation of an optical network communication system including: providing a planar lightwave circuit including: connecting 2×2 single-mode optical couplers in an array for forming a 1×N single-mode optical splitter/combiner, and routing harvesting ports to an optical line terminal receiver for collecting harvested-light, from two or more of the harvesting ports, in the optical line terminal receiver wherein one of more of the harvesting ports is from the 2×2 single-mode optical couplers; transmitting to an optical network unit through the planar lightwave circuit at a first wavelength; and interpreting a response from the optical network unit at a second wavelength through the harvested-light.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of copending U.S. patent Ser. No.12/966,795 filed on Dec. 13, 2010 to Piehler et al., entitled “OpticalNetwork Communication System With Optical Line Terminal Transreceiverand Method of Operation Thereof,” incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to an optical networkcommunication system, and more particularly to a system for providing apassive optical network.

BACKGROUND OF THE INVENTION

An example of a point-to-multipoint optical network can be the passiveoptical network. Passive optical networks are defined in standards, bywell known organizations, for general application. The network isterminated at a single point, typically located in a telecommunicationsprovider central office (CO), in an optical line terminal (OLT) and atmultiple subscriber points, typically at the subscriber's residence, byan optical network unit (ONU).

The OLT and the ONUs have single fiber interfaces which transmit andreceive optical signals at different wavelengths. The OLT transmitssignals at a wavelength λ_(down) and receives signals from the ONUs at awavelength λ_(up). The ONU transmits signals at a wavelength λ_(up) andreceives signals from the OLT at a wavelength λ_(down). The downstreamsignal broadcasts to all ONUs on the network; while upstream signalsfrom each subscriber ONU are assigned unique time slots according to atime division multiple access (TDMA) protocol.

To support high-data rates and long distances, between the OLT and ONUs,Passive Optical Networks (PONs) use single-mode optical fiber. A keycomponent in any PON is a single-mode optical splitter. The function ofa 1×N optical splitter is to split and direct identical copies of thedownstream optical signal to each of the each of the N ONU-facing ports.

The same splitter combines N upstream signals into a single, single-modeoptical port facing the OLT. The law of energy conservation requiresthat the downstream signal at each output port will be attenuated by atleast a factor of 1/N relative to the input signal. If one assumes thatall signals in the upstream are treated identically by the splitter,(i.e. the splitter has no polarization, or wavelength preferences) thena signal entering any one of the N ONU-facing ports must be attenuatedby at least a factor of 1/N by the time it reaches the single OLT-facingport, as a consequence of the second law of thermodynamics (entropycannot decrease in a closed system).

For the ideal single-mode splitter, one that has zero excess loss, thetotal downstream optical power launched into the splitter is equal tothe total power emitted from the N ONU-facing ports. For the same idealsplitter, the total optical power flowing out of the single OLT-facingupstream port can be no more than 1/N times the total optical powerlaunched into any set the N ports. A very large fraction, (N−1)/N of theupstream signal is radiated out of the single mode waveguides in thesplitter as dispersed and unusable light energy, which will be calledwaste-light.

Thus, a need still remains for an optical network communication systemwith optical line terminal transceiver that compensates for theattenuation of the upstream signal path. In view of the growth in theoptical network communication industry, world-wide, it is increasinglycritical that answers be found to these problems. In view of theever-increasing commercial competitive pressures, along with growingconsumer expectations and the diminishing opportunities for meaningfulproduct differentiation in the marketplace, it is critical that answersbe found for these problems. Additionally, the need to reduce costs,improve efficiencies and performance, and meet competitive pressuresadds an even greater urgency to the critical necessity for findinganswers to these problems.

Solutions to these problems have been long sought but prior developmentshave not taught or suggested any solutions and, thus, solutions to theseproblems have long eluded those skilled in the art.

SUMMARY OF THE INVENTION

The present invention provides a method of operation of an opticalnetwork communication system including: providing a planar lightwavecircuit including: connecting 2×2 single-mode optical couplers in anarray for forming a 1×N single-mode optical splitter/combiner, androuting harvesting ports to a receiver for collecting harvested-light,from two or more of the harvesting ports, in the receiver wherein one ofmore of the harvesting ports is from the 2×2 single-mode opticalcouplers; transmitting to an optical network unit through the planarlightwave circuit at a first wavelength; and interpreting a responsefrom the optical network unit at a second wavelength through theharvested-light.

The present invention provides an optical network communication systemincluding: a planar lightwave circuit includes: 2×2 single-mode opticalcouplers coupled in an array form a 1×N single-mode opticalsplitter/combiner, and harvesting ports routed to a receiver forcollecting harvested-light, from two or more of the harvesting ports, inthe receiver wherein one of more of the harvesting ports is from the 2×2single-mode optical couplers; an optical line terminal transmitter fortransmitting a first wavelength to an optical network unit through theplanar lightwave circuit; and a second wavelength, from the opticalnetwork unit, received through the harvested-light.

Certain embodiments of the invention have other steps or elements inaddition to or in place of those mentioned above. The steps or elementwill become apparent to those skilled in the art from a reading of thefollowing detailed description when taken with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of an optical network communicationsystem, with optical line terminal transceiver, in an embodiment of thepresent invention.

FIG. 2 (A-H) is a functional block diagram of a 2×2 single-mode opticalcouplers having a characteristic response to input wavelengths.

FIG. 3 is a functional block diagram of the 1×N single-mode opticalsplitter/combiner, of FIG. 1.

FIG. 4 is a functional block diagram of an optical line terminaltransceiver.

FIG. 5 is a schematic diagram of an optical line terminal transceiver inan embodiment of the present invention.

FIG. 6 is a functional block diagram of a passive optical networkoptical line terminal line card utilizing the planar lightwave circuitin an embodiment of the present invention.

FIG. 7 is a schematic diagram of a 1×32 single-mode opticalsplitter/combiner in an embodiment of the present invention.

FIG. 8 is a functional block diagram of a passive optical network linecard utilizing an external version of the planar lightwave circuit in asecond embodiment of the present invention.

FIG. 9 is a functional block diagram of a passive optical networkoptical line terminal line card utilizing an external version of theplanar lightwave circuit in a third embodiment of the present invention.

FIG. 10 is a functional block diagram of a passive optical networkoptical line terminal line card utilizing the planar lightwave circuitin a fourth embodiment of the present invention.

FIG. 11 is a functional block diagram of a passive optical networkoptical line terminal line card utilizing the planar lightwave circuitin a fifth embodiment of the present invention.

FIG. 12 is a schematic diagram of an optical line terminal transceiverutilizing a 1×4 single-mode optical splitter/combiner in an embodimentof the present invention.

FIG. 13 is a schematic diagram of an optical line terminal transceiverutilizing a 1×8 single-mode optical splitter/combiner in a sixthembodiment of the present invention.

FIG. 14 is a therein is shown a schematic diagram of an optical lineterminal transceiver utilizing a 2×8 single-mode opticalsplitter/combiner in a seventh embodiment of the present invention.

FIG. 15 is a functional block diagram of a 32-port 10-Gb/s PON OLTtransceiver in an eighth embodiment of the present invention.

FIG. 16 is a functional block diagram of a 32-port 10-Gb/s PON OLT reachextension system in a ninth embodiment of the present invention.

FIG. 17 is a functional block diagram of a 32-port 10-Gb/s PON OLT reachextension system in a tenth embodiment of the present invention.

FIG. 18 is a functional block diagram of a hybrid-fiber coax opticalnetwork repeater in an eleventh embodiment of the present invention.

FIG. 19 is a functional block diagram of an optical line terminaltransceiver utilizing a 1×(N/2) single mode splitter/combiner, in atwelfth embodiment of the present invention.

FIG. 20 is a functional block diagram of a passive optical networkoptical line terminal line card utilizing the planar lightwave circuit.

FIG. 21 is a flow chart of a method of operation of an optical networkcommunication system in a further embodiment of the present invention.

DETAILED DESCRIPTION

The following embodiments are described in sufficient detail to enablethose skilled in the art to make and use the invention. It is to beunderstood that other embodiments would be evident based on the presentdisclosure, and that system, process, or mechanical changes may be madewithout departing from the scope of the present invention.

In the following description, numerous specific details are given toprovide a thorough understanding of the invention. However, it will beapparent that the invention may be practiced without these specificdetails. In order to avoid obscuring the present invention, somewell-known circuits, system configurations, and process steps are notdisclosed in detail.

The drawings showing embodiments of the system are semi-diagrammatic andnot to scale and, particularly, some of the dimensions are for theclarity of presentation and are shown exaggerated in the drawing FIGs.Similarly, although the views in the drawings for ease of descriptiongenerally show similar orientations, this depiction in the FIGs. isarbitrary for the most part. Generally, the invention can be operated inany orientation.

The same numbers are used in all the drawing FIGs. to relate to the sameelements. The embodiments have been numbered first embodiment, secondembodiment, etc. as a matter of descriptive convenience and are notintended to have any other significance or provide limitations for thepresent invention.

For expository purposes, the term “horizontal” as used herein is definedas a plane parallel to the plane or surface of the Earth, regardless ofits orientation. The term “vertical” refers to a direction perpendicularto the horizontal as just defined. Terms, such as “above”, “below”,“bottom”, “top”, “side” (as in “sidewall”), “higher”, “lower”, “upper”,“over”, and “under”, are defined with respect to the horizontal plane,as shown in the figures. The term “on” means that there is directcontact between elements. The term waste-light is defined as the lightthat is diffused from an optical junction in prior art splitters. Forpurposes of this application harvested-light is not diffused in thecurrent invention but is rather collected or redirected for use by thereceiver of the present invention. The term harvesting port is definedto be the extra port of a 2×2 single-mode optical coupler or awavelength division multiplexer that is used for collecting orredirecting the harvested-light.

Referring now to FIG. 1, therein is shown a functional block diagram ofan optical network communication system 100, with optical line terminaltransceiver 101, in an embodiment of the present invention. Thefunctional block diagram of the optical network communication system 100depicts a 1×N single-mode optical splitter/combiner 102 that has atleast two single-mode optical ports, such as a first single-mode opticalport 104 and a second single-mode optical port 106 on the Optical LineTerminal-facing side. The second single-mode optical port 106, is theinput to the 1×N single-mode optical splitter/combiner 102 and may becoupled to an optical transmitter of an optical line terminal (notshown).

In addition to the first single-mode optical port 104, at least oneadditional port 108 may be directed through a multi-port single-modegroup 110, such as a group of optical fibers or optical waveguides, isdirected toward photo-detectors 116, in the optical line terminal, forharvesting upstream light. One of the additional ports of thesingle-mode group 110, may be derived from the second single-modeoptical port 106 by wavelength division multiplexing (WDM), for example.The upstream signals in the first single-mode optical port 104 and themulti-port single-mode group 110 are derived at least partially fromcollecting the harvested-light that would otherwise be dispersed from aprior art splitter, as waste-light, may now be harvested in the 1×Nsingle-mode optical splitter/combiner 102.

Upstream signals from the first single-mode optical port 104 and themulti-port single-mode group 110 are transported toward thephoto-detectors 116. The upstream signals may traverse the multi-portsingle-mode group 110 or they may be efficiently coupled to one or moremulti-mode optical waveguides 112 with low loss. The multi-mode opticalwaveguides 112 may transport the upstream signals through opticalcouplers 114, with low loss to the upstream signals. The opticalcouplers 114 may deliver the upstream signals to one or more of thephoto-detectors 116 by a combination of the multi-mode opticalwaveguides 112 and/or the multi-port single-mode group 110.

The upstream signals from the single multi-mode optical waveguide 112are coupled to the photo-detectors 116, labeled “O/E”, having activedetection area(s), not shown, substantially larger than the square ofthe signal wavelength measured in nano-meters. The coupling of theupstream signals to the photo-detectors 116 may be achieved byproximity, refractive optics (i.e. lenses), reflective surfaces, ordiffractive optics.

If there are two or more of the photo-detectors 116, an electrical bus118 is combined by means of analog or digital circuitry (not shown). Theelectrical bus 118 may be suitable for manipulation by a processor (notshown).

The present invention can be implemented in such a way that any upstreamsignals, such as optical signals, entering at an ONU-facing port 120,which are directed to one or more of the photo-detectors 116 by morethan one distinct optical path satisfying the following requirement:

N ONU-facing optical ports 122 may form an egress path for opticalnetwork units (ONU) 124, coupled by single-mode optical fiber 126, thatare communicating through the optical network communication system 100.The combination of optical, electro-optical and electronic elementsdescribed above must be designed such that the time for an upstreamsignal from any particular one of the N of the ONU-facing optical ports122 to travel along the multiple possible distinct optical paths to theelectrical bus 118 must be “equal to each other” with a tolerancesubstantially smaller than the reciprocal of the electrical bandwidth ofthe signal.

Or, in mathematical terms, the overall design must satisfy thefollowing:

|T ₁ −T ₂|<<1/B _(e)  EQ 1

For all signal paths originating at any one particular port of the NONU-facing optical ports 122.

Where:

T₁=time to travel from any of the N of the ONU-facing optical ports 122to the electrical bus 118 via a distinct path through the multi-portsingle-mode group 110.

T₂=time to travel from that same N of the ONU-facing optical port 122 tothe electrical bus 118 via a different distinct path through themulti-port single-mode group 110.

B_(e)=the electrical signal bandwidth

The present invention places no limitations on the physical distributionof its constituent parts, so long as the design rules are preserved.Accordingly, certain embodiments may physically separate constituentparts and/or locate them in distinct modules. In the examplesillustrated below, the passive optical functions may be isolated toseparate modules.

It has been discovered that by collecting the harvested-light from the1×N single-mode optical splitter/combiner 102, a simplification of anoptical line terminal receiver 128 that interprets the electrical bus118 is possible. The simplification may translate to a reduction in costand an increase in data reliability as reflected by the reduction in thebit error ratio.

Referring now to FIG. 2 (A-H), therein is shown a functional blockdiagram of a 2×2 single-mode optical coupler 202 having a characteristicresponse to input wavelengths. The functional block diagram of the 2×2single-mode optical couplers 202 depicts that each of the 2×2single-mode optical couplers 202 includes an A port 204, a B port 206, aC port 208, and a D port 210. It is understood that the 2×2 single-modeoptical couplers 202 are optical couplers that transmit light of acertain wave length (λ) as defined below.

The 2×2 single-mode optical couplers 202 are used as a key buildingblock of the 1×N single-mode optical splitter/combiner 102, of FIG. 1.The 2×2 single-mode optical couplers 202 can be fabricated fromfused-fiber, planar lightwave circuit or bulk optical technologies. Forsuccessful implementation of the 1×N single-mode opticalsplitter/combiner 102, the 2×2 single-mode optical couplers 202 shouldfunction as an equal-sided Y-junction at the Passive Optical Network(PON) Optical Line Terminal transmitter wavelength, λ_(down). At thewavelength of the PON OLT receiver, λ_(up), the specification on thecoupling ratio is much more relaxed, since in the preferred embodiment,most or all paths, eventually get to the OLT receiver regardless of anyparticular split ratio. This is a departure from the prior art, whichdoes not collect the harvested-light as defined in this application.

It is known by those skilled in the art that designing and fabricating aPlanar Lightwave Circuit based on broadband features of the 2×2single-mode optical couplers 202 is more challenging than one where thecoupler is designed to split power equally over a narrow range ofwavelengths. In this application, downstream performance is morecritical than upstream performance. The basic definition of the 2×2single-mode optical couplers 202 in the application are illustratedbelow:

As shown in FIG. 2(A), the power of the transmitted light P_(λ(Tx))entering the A port 204 of the 2×2 single-mode optical coupler 202 isequally divided and replicated on both the C port 208, and the D port210. Each of the destination ports will propagate the light at ½P_(λ(Tx)).

As shown in FIG. 2(B), the power of the transmitted light P_(λ(Tx))entering the B port 206 of the 2×2 single-mode optical coupler 202 isequally divided and replicated on both the C port 208, and the D port210. Each of the destination ports will propagate the light at ½P_(λ(Tx)).

As shown in FIG. 2(C), the power of the transmitted light P_(λ(Tx))entering the C port 208 of the 2×2 single-mode optical coupler 202 isequally divided and replicated on both the A port 204, and the B port206. Each of the destination ports will propagate the light at ½P_(λ(Tx)).

As shown in FIG. 2(D), the power of the transmitted light P_(λ(Tx))entering the D port 210 of the 2×2 single-mode optical couplers 202 isequally divided and replicated on both the A port 204, and the B port206. Each of the destination ports will propagate the light at ½P_(λ(Tx)). As shown in FIG. 2(E), the power of the received lightP_(λ(Rx)) entering the A port 204 of the 2×2 single-mode opticalcouplers 202 is arbitrarily divided between the C port 208, and the Dport 210. Each of the destination ports will propagate a complimentaryportion the light at P1 and P2, where:

P1+P2=P _(λ(Rx))  EQ 2

with

P1≧0.05P _(λ(Rx))  EQ 3

and

P2≧0.05P _(λ(Rx))  EQ 4

As shown in FIG. 2(F), the power of the received light P_(λ(Rx))entering the B port 206 of the 2×2 single-mode optical couplers 202 isarbitrarily divided between the C port 208, and the D port 210. Each ofthe destination ports will propagate a complimentary portion the lightat P1 and P2, as defined above.

As shown in FIG. 2(G), the power of the received light P_(λ(Rx))entering the C port 208 of the 2×2 single-mode optical couplers 202 isarbitrarily divided between the A port 204, and the B port 206. Each ofthe destination ports will propagate a complimentary portion the lightat P1 and P2, as defined above.

As shown in FIG. 2(H), the power of the received light P_(λ(Rx))entering the D port 210 of the 2×2 single-mode optical couplers 202 isarbitrarily divided between the A port 204, and the B port 206. Each ofthe destination ports will propagate a complimentary portion the lightat P1 and P2, as defined above.

Referring now to FIG. 3, therein is shown a functional block diagram ofthe 1×N single-mode optical splitter/combiner 102, of FIG. 1. Thefunctional block diagram of the 1×N single-mode opticalsplitter/combiner 102 depicts an array 302 of the 2×2 single-modeoptical couplers 202. A primary input is an OLT-facing port 304, whichmay be coupled to an OLT transmitter (not shown).

In the prior art, an ideal 1×N single mode optical splitter might bemade of Y-junctions and is perfectly efficient in the downstreamdirection. However in the upstream direction, at each Y-junction, onlyone-half of the power from each leg will couple into the single upstreamwaveguide (not shown), while the excess power will radiate out of thewaveguide as waste-light.

In the present invention each of the Y-junctions is replaced by abroadband version of the 2×2 single-mode optical couplers 202. The totalupstream power launched into the two downstream facing legs can bepreserved in the two upstream legs without radiating any portion of thepower out from the waveguides. Only one of the two OLT-facing legs isused to form the 1×N single-mode optical splitter/combiner 102, as shownin FIG. 3.

An ideal 1×N single-mode optical splitter/combiner 102 may be formed outof (N−1) of the 2×2 single-mode optical couplers 202. In the presentexample, with N=8, a 1×8 single-mode optical splitter/combiner 300 maybe formed by coupling 7 of the 2×2 single-mode optical couplers 202. Itis understood that the selection of the number 8 for the 1×N single-modeoptical splitter/combiner 102 is an example only and is not used in alimiting manner. It is also understood that the present invention may bepracticed by using any number of the 2×2 single-mode optical couplers202.

The total power launched into any subset of the ONU-facing optical ports122 is described as P and the power from the OLT-facing port 304 will beP/N and the sum of the optical power from harvesting ports 306 labeledA, B, C . . . G will total P (N−1)/N. Also note that the 1×N single-modeoptical splitter/combiner 102 can be formed from (N−1) of the 2×2single-mode optical couplers 202, each with one port uncommitted andavailable to become the harvesting ports 306. In the above example whereN was chosen to be 8, it takes (N−1) or 7 of the 2×2 single-mode opticalcouplers 202 to implement a 1×8 single-mode splitter/combiner 300.

Referring now to FIG. 4, therein is shown a functional block diagram ofan optical line terminal transceiver 400 in an embodiment of the presentinvention. The functional block diagram of the optical line terminaltransceiver 400 depicts the 1×N single-mode optical splitter/combiner102, of FIG. 1, coupled to form the 1×8 single-mode splitter/combiner300 of the previous example.

An array of the ONU-facing optical ports 122 may be coupled to opticalnetwork units (ONU) 124, of FIG. 1, that may be coupled by thesingle-mode optical fiber 126, of FIG. 1, and placed a great distanceaway from the optical line terminal transceiver 400. The array of theONU-facing optical ports 122 is shown as not connected for simplicity ofthe description and it is understood that in an operational environmentthe single-mode optical fiber 126 and the optical network units 124, ofFIG. 1, would be present.

The present invention utilizes the optical power from the harvestingports 306 (A through G) of the 2×2 single-mode optical couplers 202,which make up the 1×N single-mode optical splitter/combiner 102 in aformat usable for the optical communications system.

An ideal model of a wavelength division multiplexer 404, such as a threeport wavelength division multiplexer, is attached to the OLT-facing port304 of the 1×8 single-mode splitter/combiner 300. The harvesting port306, of the wavelength division multiplexer 404, diverts the upstreamwavelength (λ(Rx)) that is identified as A′, while the path from anoptical line terminal-facing port 418 to first coupler via thewavelength division multiplexer 404 is fully transmissive at thedownstream wavelength (λ(Tx)) as sourced from an optical line terminaltransmitter 410.

The full recovery of the upstream power, P_(λ(Rx)), is made possible bycoupling the harvesting ports 306, (A through G) from the 2×2single-mode optical couplers 202 and (A′) from the wavelength divisionmultiplexer 404 to collect harvested-light from the harvesting ports306, through an optical/electrical converter 412 to the optical lineterminal receiver 128. The harvested-light collected from the harvestingports 306 of the 2×2 single-mode optical couplers 202 would, in priorart systems, normally be allowed to dissipate within the splitter aswaste-light without being used.

By collecting the harvested-light in the optical line terminal receiver128, the present invention simplifies the receiving process and collectsall of the power delivered to the 2×2 single-mode optical couplers 202.In this case, if the total power launched into any subset of the N portsis P, the power from the optical line terminal-facing port 418 will bezero and the sum of the optical power from the harvesting ports 306,labeled A, B, C . . . G and A′, will total P.

It has been discovered that the present invention may simplify thedesign requirements of an optical line terminal receiver electronics 414by providing the full amount of the P_(λ(Rx)) that was initiallylaunched. The simplification in the optical line terminal receiverelectronics 414 may reduce cost of the optical line terminal receiverelectronics 414 while increasing performance by decreasing the bit errorratio of the receiver data 416 that was launched data from the ONU 124,of FIG. 1.

In a passive optical network (PON) optical line terminal the opticalline terminal (OLT) transmitter 410 generates a downstream opticalsignal at the wavelength λ(Tx). The optical signal from the PON OLTtransmitter 410 is directed through the optical line terminal-facingport 418, such as an OLT-facing single-mode optical port. At the PON OLTthere is the optical line terminal receiver 128 designed to receiveoptical signals from the ONUs at the wavelength λ_((Rx)).

The invention can be used to couple light from the harvesting ports 306,of FIG. 3, of the 2×2 single-mode optical couplers 202, which comprisethe 1×N single-mode optical splitter/combiner 102, and the harvestingport 306 of the wavelength division multiplexer 404 to the optical lineterminal receiver 128 such that the upstream optical signal from any oneof the ONU-facing optical ports 122 is simultaneously directed to theoptical line terminal receiver 128 by two or more distinct opticalpaths, in a format usable for the optical communications system.

To insure a format usable for the optical communications, the opticalsignals in at least two of the harvesting ports 306 (originating at theharvesting ports 306 of the 2×2 single-mode optical couplers 202 and theharvesting port 306 of the wavelength division multiplexer 404) arecoupled into one or more single-mode or multi-mode optical waveguides orfibers (indicated by the dashed lines) and one or more of theoptical/electrical converter 412, in which the active area of aphoto-detector (not shown) is significantly larger than the mode fielddiameter of single-mode optical signal in the single-mode opticalwaveguide.

The format usable for the optical communications system requires thatthe time for the communications signal to propagate from any of thearray of the ONU-facing optical ports 122 to a common electricaljunction within the receiver through multiple optical paths aresubstantially equal within a tolerance of Δt, where Δt<<1/B_(e) andB_(e) is the electrical bandwidth of the communication signal modulatingthe optical carrier.

Referring now to FIG. 5, therein is shown a schematic diagram of anoptical line terminal transceiver 500 in an embodiment of the presentinvention. The schematic diagram of the optical line terminaltransceiver 500 depicts a planar lightwave circuit 502 having a 1×Nsingle-mode optical splitter/combiner with the 2×2 single-mode opticalcouplers 202 arranged to provide a 1×4 single-mode opticalsplitter/combiner as an example of the present invention.

It is understood that the 1×4 single-mode optical splitter/combiner isan example to aid in the discussion of the present invention and notintended to limit the range of the invention in any way.

The preferred embodiment, as an example, integrates this invention intothe structure of the planar lightwave circuit 502. The four-port versionintegrated onto the planar lightwave circuit 502 utilizes the 2×2single-mode optical couplers 202 that are designed to evenly split powerfrom the optical line terminal transmitter 410 at a wavelength,λ_((Tx)).

In this case a first wavelength 503, such as 1480 nm≦λ_((Tx))≦1500 nm asis required for Gigabit Passive Optical Network (GPON) and GigabitEthernet Passive Optical Network (GE-PON) systems. The wavelengthdivision multiplexer 404 preferentially directs some of the upstreamsignal at a second wavelength 512, such as 1260 nm≦λ_(Rx)≦1360 nm, to anavalanche photo diode 504, while minimally impacting the flow of thedownstream signal through the wavelength division multiplexer 404.

The planar lightwave circuit 502 structure is designed such that anypath from any one of the four ports in the array of the ONU-facingoptical ports 122 to the active surface of the avalanche photo diode 504are equal within a tolerance of 40 pico-seconds (ps), in order toprovide the format usable for the optical communications system, assuitable to enable “on-off keying”—“non-return to zero” (OOK-NRZ)signals at 1.25 Gb/s as are used in the upstream of a GE-PON or GPON.The designer must take into account not only the path lengths of thewaveguides on the planar lightwave circuit 502, but also the modaldispersion of a multi-mode optical waveguide 506, in determining theoptimal design.

In the preferred embodiment of the planar lightwave circuit 502,single-mode optical waveguides 508, such as single-mode opticalwaveguides, have a numerical aperture (NA) of 0.22, a cross-section of4.5-μm square, and the mode field diameter of approximately 3.7 μm at1310 nm Single-mode optical waveguides 508, such as patternedpoly-silicon glass or optical fiber, may be routed to the exit of theplanar lightwave circuit 502 in a array of single-mode opticalwaveguides 510 with 8 μm spacing. The harvested-light (λ_(H)) 514 may bederived from the second wavelength 512 received through the array of theONU-facing optical ports 122.

The multi-mode optical waveguide 506 has a 50-μm core diameter and a NAof 0.27. The single-mode optical waveguides 508 described cansimultaneously be coupled to the multi-mode optical waveguide 506 withnearly unity efficiency. The opposite end of the multi-mode opticalwaveguide 506 may be pigtailed to the avalanche photo diode 504 with a65-μm active area at an efficiency approaching unity.

The design of the single-mode optical waveguides 508 and choice of themulti-mode optical waveguide 506, including the length of the multi-modeoptical waveguide 506, must insure that the signal from any one of thearray of the ONU-facing optical ports 122, propagating along distinctoptical paths, must arrive at the avalanche photo diode 504 within atolerance of 40 ps with any other copies of the signal originating fromthe identical one of the ONU-facing optical ports 122, but propagatingalong a distinct optical path. The total delay from a specific one ofthe ONU-facing optical ports 122 through any of the single-mode opticalwaveguides 508, to the avalanche photo diode 504 also includes modaldispersion in the multi-mode optical waveguide 506.

The schematic diagram of FIG. 5 and other diagrams in this specificationare an example only and are not intended to convey design criteria ofthe single-mode optical waveguides 508 but only to show linkages withoutproviding additional limitations. The actual layout of the single-modeoptical waveguides 508 must conform to the multi-path lengthrestrictions dictated by the application in order to meet the criteriaof the present invention.

The planar lightwave circuit 502 is an integrated device, which can beused to split the downstream optical signal from the optical lineterminal transmitter 410, in an implementation of a GE-PON or GPON OLT,into the array of the ONU-facing optical ports 122, while simultaneouslycombining the upstream optical power from the array of the ONU-facingoptical ports 122 and guiding the upstream signal to the avalanche photodiode 504 in a form useful for upstream communications at the bit rateconforming to the GE-PON and GPON standards.

Referring now to FIG. 6, therein is shown a functional block diagram ofa passive optical network optical line terminal line card 600 utilizingthe planar lightwave circuit 502 in an embodiment of the presentinvention. The functional block diagram of the passive optical networkoptical line terminal line card 600 depicts a small form-factorpluggable mechanical interface 602 having an electrical interface 606,such as a small form-factor pluggable industry standard interface forpassive optical network support. The electrical interface 606 has anindustry accepted specification for mechanical and electricaltolerances.

A small form-factor pluggable module 608 may couple to the mechanicalinterface 602, and to the electrical interface 606, through itselectrical interface 604. In an embodiment of the present invention, asan example, the small form-factor pluggable module 608 may include anoptical line terminal electrical circuit 610 that manages thetransmission and receiving of the data between the small form-factorpluggable module 608 and the optical network units 124, of FIG. 1, thatare distributed along the optical network (not shown). The optical lineterminal electrical circuit 610 may provide an electrical interface thatdrives the optical line terminal transmitter 410, such as a distributedfeedback (DFB) laser, and receives an electrical signal from theavalanche photo diode 504.

The optical line terminal electrical circuit 610 may condition the datato transfer it into the small form-factor pluggable module 608 in a TimeDivision Multiplexing (TDM) data format rather than the Time DivisionMultiple Access (TDMA) format that is exchanged through the array of theONU-facing optical ports 122. The presence of the multi-mode opticalwaveguide 506 connection from the planar lightwave circuit 502 to theavalanche photo diode 504 allows the inclusion of the harvesting ports306 of the 2×2 single-mode optical couplers 202 where only one of theoptical line terminal-facing port 304 is supported by the prior art. Byproviding the planar lightwave circuit 502 on the small form-factorpluggable module 608, the number of the ONU-facing optical ports 122 isincreased from one to four without increasing the space or hardwarerequired by the prior art solutions. It is understood that the planarlightwave circuit 502 is shown having four of the array of theONU-facing optical ports 122 by way of an example and other numbers ofthe ONU-facing optical ports 122 is possible.

It will be understood by those skilled in the art that the invention canbe extended to other types of point-to-multi-point opticalcommunications networks, including but not limited those defined by the10G-EPON standard, the emerging ITU-T 10G-PON standard, RF over glass(RFoG) networks, other hybrid-fiber coax networks, and otherpoint-to-multi-point optical (or partially optical) networks.

While the above example includes the multi-mode optical waveguide 506connection through the avalanche photo diode 504, other implementationsare possible that do not include the multi-mode optical waveguide 506.As an example, each of the single-mode optical waveguides 508 from theplanar lightwave circuit 502 may be coupled to individual single-modereceivers (not shown) for conversion to the electrical interface of theoptical line terminal electrical circuit 610.

The planar lightwave circuit 502 of the present invention may haveapplication as a broadband downstream splitter, which is universal inoptical broadcast communication networks. Alternate wavelengths can beincorporated into the invention for altering the intended application.The present invention may enable an implementation that is a broadbandsingle-mode splitter in downstream direction, and awavelength-independent optical power combiner in the upstream direction.The resulting device is applicable to various known passive opticalnetwork (PON) and hybrid-fiber coax networks (HFC) implementations,without regard for wavelength choice, and is only limited by bandwidthrequirements on the upstream signal, and the tolerance on the multipathcombining optics, electro-optics, and electronics.

It is understood that the planar lightwave circuit 502 may includeactive optical elements (not shown) such as the optical line terminaltransmitter 410 or receivers with optical booster or pre-amplifiers, theavalanche photo diode 504, optical intensity amplifiers, polarization orphase modulators, optical amplifiers including semiconductor opticalamplifiers, or variable optical attenuators. Other implementations caninclude a photo-receiver whose active area has a shape more compatiblewith the linear output of a multi-mode slab waveguide or the array ofsingle-mode optical waveguides 510 of the single-mode optical waveguides508.

The planar lightwave circuit 502 can also include integrated passiveoptical elements such thin-film filters, Mach-Zehnder-basedinterferometric filters, arrayed waveguide gratings, Bragg gratings, ormulti-mode interference filters. One critical use for passive filters isto keep any stray light from the optical line terminal transmitter 410or other sources from interfering with the received signal at theavalanche photo diode 504.

Referring now to FIG. 7, therein is shown a schematic diagram of a 1×32single-mode optical splitter/combiner 700 in an embodiment of thepresent invention. The schematic diagram of the 1×32 single-mode opticalsplitter/combiner 700 depicts 31 of the 2×2 single-mode optical couplers202 coupled in a pyramid fashion.

While the preferred embodiment of the 1×32 single-mode opticalsplitter/combiner 700 is shown to include the 2×2 single-mode opticalcouplers 202, various constructions may be accommodated and built as theplanar lightwave circuit 502, of FIG. 5, including but not limited toarrayed waveguide gratings (AWGs), multi-mode interferometers (MMIs) andsingle-mode star couplers.

In general, any single-mode N×N star coupler structure, including (butnot limited to) Dragone routers, MMIs, arrays of the 2×2 single-modeoptical couplers 202, and fused-fiber based star couplers can functionas both a 1×N splitter, and as a collector for the harvested-light. Ifthe waveguide design can accommodate the restrictions on the path lengthtolerances required by the upstream bandwidth, any N×N star couplerstructure can form the basis or a part of an implementation.

In implementing the planar lightwave circuit 502, care must be taken tolimit the number of crossings of the single-mode optical waveguides 508because each of the crossings of the single-mode optical waveguides 508increases an optical loss penalty, and decreases manufacturability.

The embodiment of the 1×32 single-mode optical splitter/combiner 700 hasa total of 24 waveguide crossings, and the worst case path from any ofthe ONU-facing optical ports 122 to the upstream photo-detector includesat most two waveguide crossings. This represents a significantimprovement over the prior art, which may provide reduced size andimproved manufacturing margins.

Integration of a 1×32 single-mode optical splitter with a multi-pathoptical power combiner there are a total of 36 waveguide crossingscompared to 496 for the prior art, and the worst case path crossing is26 on the splitter, and the worst case on the combiner crossing is two,compared to the prior art which requires 31 optical crossings for thesplitter and 31 optical crossings for the combiner. The significantreduction in the number of optical crossings simplifies the design ofthe planar lightwave circuit 502 and increases the manufacturing margin.

An additional benefit is the lowering or elimination of wavelengthdependence. In the PON application, both upstream and downstreamwavelengths are defined over a specified range and the planar lightwavecircuit 502 can accommodate the entire range with no additional changes.

By way of an example, GPONs require that the optical line terminaltransmitter 410 (downstream) emits light at a wavelength between 1480and 1500 nm. All ONU transmitters (upstream) must emit light at awavelength between 1260 and 1360 nm. It will be understood by one havingordinary skill in the art that the 2×2 single-mode optical couplers 202and the single-mode optical waveguides 508 are capable of supportingboth ranges concurrently.

It is understood that as N increases; the value of including awavelength division multiplexer coupler 404 to harvest the upstreamlight from the OLT-facing port 304 diminishes. In addition, therequirement that the 2×2 single-mode optical couplers 202 operate over abroad band is relaxed. There is no requirement on the optical powersplit ratio in the upstream, only the downstream splitting requirement(i.e. 50% to each port of the 2×2 single-mode optical couplers 202)needs specification. It is known by those having ordinary skill in theart that more care and space is required in creating a planar lightwavecircuit-based 2×2 single-mode optical couplers 202 that operates over abroad band than one that operates over a narrow band.

In addition, the fabrication of planar lightwave circuit-based WDMfilters becomes more difficult and requires more sub-stages, whichtranslates into more total planar lightwave circuit length, aswavelength specifications are tightened. The prior art requires Nthree-port WDM couplers. In comparison the present invention requireszero or one of the three-port WDM couplers simplifying both design andmanufacturability, while also reducing device size.

The elimination of the WDM couplers from the 1×32 single-mode opticalsplitter/combiner 700 makes the device much more wavelength agnostic.One having ordinary skill in the art can now design a single device tooperate under a wide range of wavelength requirements. Thisone-size-fits-all approach can lead to lower manufacturing complexity,and shorter time-to-market for a new product.

Referring now to FIG. 8, therein is shown is a functional block diagramof a passive optical network optical line terminal line card 800utilizing an external version of the planar lightwave circuit 502 in asecond embodiment of the present invention. The functional block diagramof the passive optical network optical line terminal line card 800depicts a small form-factor pluggable module 802 having the primaryinterface 604, such as a small form-factor pluggable industry standardinterface for passive optical network support. The primary interface 604has an industry accepted specification for mechanical and electricaltolerances.

A Passive Optical Network mechanical interface 804 can have theelectrical interface 606. In the prior art structure of a pluggableoptical module (not shown) that is substantially similar to the smallform-factor pluggable module 802, only one of the ONU-facing opticalports 122 was provided. This prior art limitation causes additionalhardware, space, and power to be consumed in order to increase thenumber of the ONU-facing optical ports 122 supported by the passiveoptical network line card 800.

In the second embodiment of the present invention, the small form-factorpluggable module 802 may include the optical line terminal electricalcircuit 610 that manages the transmission and receiving of the databetween the small form-factor pluggable module 802 and the opticalnetwork units 124, of FIG. 1, that are distributed along the opticalnetwork (not shown). The optical line terminal electrical circuit 610can provide an electrical interface that drives the optical lineterminal transmitter 410, such as a distributed feedback (DFB) laser,and receives an electrical signal from the avalanche photo diode 504.The small form-factor pluggable module 802 may be shorter than the smallform-factor pluggable module 608, of FIG. 6, because the planarlightwave circuit 502 is moved to a remote interface board 806, such asa completely passive optical interface board.

It is understood that the optical and electrical contents of the smallform-factor pluggable module 802 may be assembled in other form factorsand the use of the small form-factor pluggable module 802 as an exampleis provided because of the challenging nature of the small size. It isfurther understood that the remote interface board 806 is a completelypassive optical element that may be used to extend or expand an existingoptical network.

By providing the planar lightwave circuit 502 on the remote interfaceboard 806, the number of the ONU-facing optical ports 122 is increasedfrom one to four without increasing the space or hardware required bythe prior art solutions. It is understood that the planar lightwavecircuit 502 is shown having four of the ONU-facing optical ports 122 byway of an example and other numbers of the ONU-facing optical ports 122is possible. It is also understood that the remote interface board 806may provide interconnect hardware for coupling the multi-mode opticalwaveguide 506 and the single-mode fibers from the optical network units124, of FIG. 1.

It is further understood that the present invention places nolimitations on the physical location of its constituent parts, so longas the design rules are preserved. Accordingly, certain embodiments mayadvantageously physically separate constituent parts and/or locate themin distinct modules. In the examples illustrated below, the passiveoptical functions are isolated to the remote interface board 806.

Referring now to FIG. 9, therein is shown a functional block diagram ofa passive optical network optical line terminal line card 900 utilizingthe external version of the planar lightwave circuit 502 in a thirdembodiment of the present invention. The functional block diagram of thepassive optical network optical line terminal line card 900 depicts thesmall form-factor pluggable module 902 having the primary interface 604,such as a small form-factor pluggable industry standard interface forpassive optical network support. The primary interface 604 has anindustry accepted specification for mechanical and electricaltolerances. The Passive Optical Network mechanical interface 804 mayhave the electrical interface 606.

In the third embodiment of the present invention, the small form-factorpluggable module 902 may include the optical line terminal electricalcircuit 610 that manages the transmission and receiving of the databetween the small form-factor pluggable module 902 and the opticalnetwork units 124, of FIG. 1, that are distributed along the opticalnetwork (not shown). The optical line terminal electrical circuit 610may provide an electrical interface that drives the optical lineterminal transmitter 410, such as a distributed feedback (DFB) laser,and receives an electrical signal from the avalanche photo diode 504.

The small form-factor pluggable module 902 may be smaller than the smallform-factor pluggable module 608, of FIG. 6, because the planarlightwave circuit 502 is moved to the remote interface board 904, suchas a completely passive optical interface board. A single-mode interfacebus 906 may comprise a bundle of single mode fibers coupled between theremote interface board 904 and the small form-factor pluggable module902. A single-mode to multi-mode combiner 908 is a multiple inputsingle-mode converter to a single output of the multi-mode opticalwaveguide 506. The single-mode to multi-mode combiner 908 may include alens structure, a proximity structure or the like.

By providing the planar lightwave circuit 502 on the remote interfaceboard 904, the number of the ONU-facing optical ports 122 is increasedfrom one to four without increasing the space or hardware. It isunderstood that the planar lightwave circuit 502 is shown having four ofthe ONU-facing optical ports 122 by way of an example and other numbersof the ONU-facing optical ports 122 is possible. It is also understoodthat the remote interface board 806 may provide interconnect hardwarefor coupling the single-mode interface bus 906 and the single-modefibers from the optical network units 124, of FIG. 1. A single-modeoptical fiber 910 may be coupled between the optical line terminaltransmitter 410 and the remote interface board 904.

Referring now to FIG. 10, therein is shown a functional block diagram ofa passive optical network optical line terminal line card 1000 utilizingthe planar lightwave circuit in a fourth embodiment of the presentinvention. The functional block diagram of the passive optical networkoptical line terminal line card 1000 depicts the small form-factorpluggable module 608 having the primary interface 604, such as a smallform-factor pluggable industry standard interface for passive opticalnetwork support. The primary interface 604 has an industry acceptedspecification for mechanical and electrical tolerances.

The small form-factor pluggable mechanical interface 602 may have theelectrical interface 606. The electrical interface 606 is intended toallow replacement of the small form-factor pluggable module 608.

In a fourth embodiment of the present invention, the small form-factorpluggable module 608 may include the optical line terminal electricalcircuit 610 that manages the transmission and receiving of the databetween the small form-factor pluggable module 608 and the opticalnetwork units 124, of FIG. 1, that are distributed along the opticalnetwork (not shown). The optical line terminal electrical circuit 610may provide the electrical interface that drives the optical lineterminal transmitter 410, such as a distributed feedback (DFB) laser,and receives the electrical signal from the avalanche photo diode 504.

By providing a planar lightwave circuit 1002 on the small form-factorpluggable module 608, the number of the ONU-facing optical ports 122 isincreased from one to four without increasing the space or hardwarerequired by the prior art solutions. It is understood that the planarlightwave circuit 1002 is shown having four of the ONU-facing opticalports 122 by way of an example and other numbers of the ONU-facingoptical ports 122 is possible.

The planar lightwave circuit 1002 may provide a utility port 1004 forattachment of an optical time domain reflectometer 1006. The opticaltime domain reflectometer 1006 may transmit and receive networkmonitoring signals without adding any additional signal degradationpenalties. In the prior art configuration, application of an opticaltime domain reflectometry probe wavelength was accomplished by theaddition of a WDM filter, adding a non-negligible insertion loss to theoverall PON link budget. Use of a low-bend loss optical fiber betweenthe planar lightwave circuit 1002 and the utility port 1004 facilitatesthe circuitous optical path with minimum insertion loss. In this examplethe optical time domain reflectometer 1006 is shown as an externaldevice which may be inserted for diagnostic purposes.

The utility port 1004 may also be advantageously used for injection ofan overlay wavelength for broadcast (one-way) services, such as the1550-1560 nm video enhancement band defined in both IEEE and ITU-Tstandards. The injection of the overlay wavelength through the utilityport 1004 does not impose any additional signal loss due to theconnection of the utility port 1004.

The utility port 1004 may also be used as an input port for a nextgeneration overlay for two-way passive optical networks on the existingpassive optical network infrastructure. In many cases a blocking filtermay be inserted before the photo-detector or detectors to eliminateinterference from the new PON upstream signals onto the old PON upstreamsignals. An example of this would be the overlay line of the“next-generation” G.987 10G-rate PON on an existing G.984 GPON. In theprior art, and as defined in draft versions of G.987.2, such an overlaywould contribute a 1 dB additional loss to the PON link budget due tothe insertion loss of the WDM filter. Notably, in the implementation ofthe present invention this 1 dB loss is eliminated.

In the example above, one of the ports from the most common of the 2×2single-mode optical couplers 202 is presented to the edge of the modulefor reuse. In an identical fashion, a broadcast video network or anext-generation PON can be applied to the existing PON by utilizing thesame port.

Referring now to FIG. 11, therein is shown a functional block diagram ofa passive optical network optical line terminal line card 1100 utilizingthe planar lightwave circuit 1002 in a fifth embodiment of the presentinvention. The functional block diagram of the passive optical networkoptical line terminal line card 1100 depicts a small form-factorpluggable module 1102 having the primary interface 604, such as a smallform-factor pluggable industry standard interface for passive opticalnetwork support. The primary interface 604 has an industry acceptedspecification for mechanical and electrical tolerances.

In the fifth embodiment of the present invention, the small form-factorpluggable module 1102 may include the optical line terminal electricalcircuit 610 that manages the transmission and receiving of the databetween the small form-factor pluggable module 1102 and the opticalnetwork units 124, of FIG. 1, that are distributed along the opticalnetwork (not shown). The optical line terminal electrical circuit 610may provide the electrical interface that drives the optical lineterminal transmitter 410, such as a distributed feedback laser, andreceives the electrical signal from the avalanche photo diode 504.

The presence of the multi-mode optical waveguide 506 connection throughthe avalanche photo diode 504 allows the inclusion of four of theONU-facing optical ports 122 where only one of the ONU-facing opticalports 122 is supported by the prior art.

By providing the planar lightwave circuit 1002 on the small form-factorpluggable module 608, the number of the ONU-facing optical ports 122 isincreased from one to four without increasing the space or hardwarerequired by the prior art solutions. It is understood that the planarlightwave circuit 1002 is shown having four of the ONU-facing opticalports 122 by way of an example and other numbers of the ONU-facingoptical ports 122 is possible.

The planar lightwave circuit 1002 may provide a utility link 1104 forattachment of an optical time domain reflectometer probe 1106, such as apassive receiver probe or a bi-directional transceiver probe, locatedwithin the small form-factor pluggable module 1102. The optical timedomain reflectometer probe 1106 may receive network monitoring signalswithout adding any additional signal degradation penalties to the SFPPON OLT transceiver.

Referring now to FIG. 12, therein is shown a schematic diagram of anoptical line terminal transceiver 1200 utilizing a 1×4 single-modeoptical splitter/combiner in an embodiment of the present invention. Theschematic diagram of the optical line terminal transceiver 1200 depictsthe planar lightwave circuit 502 having the 2×2 single-mode opticalcouplers 202 arranged to provide the 1×4 single-mode opticalsplitter/combiner in an example of the present invention.

The preferred embodiment, as an example, integrates this invention intothe structure of the planar lightwave circuit 502. The four-port versionintegrated onto the planar lightwave circuit 502 utilizes the 2×2single-mode optical couplers 202 that are designed to evenly split powerfrom the optical line terminal transmitter 410 at the wavelength,λ_((Tx)). In this case, 1480 nm≦λ_((Tx))≦1500 nm as is required forGigabit Passive Optical Network (GPON) and Gigabit Ethernet PassiveOptical Network (GE-PON) systems. The wavelength division multiplexer404 preferentially directs some of the upstream signal at 1260nm≦λ_(Rx)≦1360 nm to the optical line terminal receiver 128, which maycontain a combination of optical and electrical devices (not shown),without impacting the flow of the downstream signal through thewavelength division multiplexer 404.

The planar lightwave circuit 502 structure is designed such that anypath from any one of the four ports in the array of the ONU-facingoptical ports 122 to the optical line terminal receiver 128 are equalwithin a tolerance of 40 ps, which is derived from the example above, assuitable to enable “on-off keying”—“non-return to zero” (OOK-NRZ)signals at 1.25 Gb/s as are used in the upstream direction of a GE-PONor GPON. The implementation must take into account not only the pathlengths of the waveguides on the planar lightwave circuit 502, but alsothe modal dispersion of an optical coupler 1202, such as a lens, arefractive coupler, a reflective coupler, or a proximity device, indetermining the optimal design.

In the preferred embodiment of the planar lightwave circuit 502, thesingle-mode optical waveguides 508 have the numerical aperture (NA) of0.22, a cross-section of 4.5-μm square, and the mode field diameter ofapproximately 3.7 μm at 1310 nm. The single-mode optical waveguides 508are routed to the exit of the planar lightwave circuit 502 in the arrayof single-mode optical waveguides 510 with 8 μm spacing. The opticalcoupler 1202, such as a lens, a refractive coupler, a reflectivecoupler, a proximity device, or the like. Those having ordinary skill inthe art will realize that all four of the single-mode optical waveguides508 described can simultaneously be coupled to the optical line terminalreceiver 128 with a 65-μm active area at an efficiency approachingunity.

The design of the single-mode optical waveguides 508 and choice of theoptical coupler 1202, must insure that the signal from any one of theONU-facing optical ports 122, must arrive at the optical line terminalreceiver 128, through two or more distinctive optical paths, within atolerance of 40 ps with any other of the ONU-facing optical ports 122.The delay through any of the single-mode optical waveguides 508 mustalso consider any modal dispersion in the optical coupler 1202 due tothe wavelength of the incoming signal.

The planar lightwave circuit 502 is an integrated device, which can beused to split the downstream optical signal from the optical lineterminal transmitter 410, in an implementation of a GE-PON or GPON OLT,into the ONU-facing optical port 122, while simultaneously combining theoptical power from the ONU-facing optical port 122 and guiding theupstream signal to the optical line terminal receiver 128 in a formuseful for upstream communications at the bit rate conforming to thepassive optical network standards.

Referring now to FIG. 13, therein is shown a schematic diagram of anoptical line terminal transceiver 1300 utilizing a 1×8 single-modeoptical splitter/combiner in a sixth embodiment of the presentinvention. The schematic diagram of the optical line terminaltransceiver 1300 depicts a planar lightwave circuit 1302 having the 2×2single-mode optical couplers 202 arranged to provide the 1×8 single-modeoptical splitter/combiner having a split receiver path as an example ofthe present invention.

The sixth embodiment integrates this invention into the structure of theplanar lightwave circuit 1302. The eight-port version integrated ontothe planar lightwave circuit 1302 utilizes the 2×2 single-mode opticalcouplers 202 that are designed to evenly split power from the opticalline terminal transmitter 410 at the wavelength, λ_((Tx)). In thisexample, 1480 nm≦λ_((Tx))≦1500 nm as is required for Gigabit PassiveOptical Network (GPON) and Gigabit Ethernet Passive Optical Network(GE-PON) systems. The wavelength division multiplexer 404 preferentiallydirects some of the upstream signal at 1260 nm≦λ_(Rx)≦1360 nm to theavalanche photo diode 504, without impacting the flow of the downstreamsignal through the wavelength division multiplexer 404.

The planar lightwave circuit 1302 structure is designed such that anypath from any one of the first four ports in the array of the ONU-facingoptical ports 122 to the optical line terminal receiver 128, travellingthrough the optical coupler, are equal within a tolerance of 40 ps,which is derived from the timing requirements of the data stream, assuitable to enable “on-off keying”—“non-return to zero” (OOK-NRZ)signals at 1.25 Gb/s as are used in the upstream direction of a GE-PONor GPON.

The implementation must take into account not only the path lengths ofthe waveguides on the planar lightwave circuit 1302, but also the modaldispersion of the first multi-mode optical waveguide 1304 and the secondmulti-mode optical waveguide 1308, in determining the optimal design. Inthis multiple receiver environment the delay path for any of the bitsfrom the array of the ONU-facing optical ports 122 should meet the 40 psdesign tolerance.

In the sixth embodiment of the invention, the planar lightwave circuit1302 may have the single-mode optical waveguides 508 with the numericalaperture (NA) of 0.22, a cross-section of 4.5-μm square, and the modefield diameter of approximately 3.7 μm at 1310 nm. The single-modeoptical waveguides 508 exit the planar lightwave circuit 1302 in twolinear arrays with 8 μm spacing. The optical coupler 1202 may include alens, reflective coupling, refractive coupling, proximity coupling, orthe like. Those having ordinary skill in the art will realize that allfour of the single-mode optical waveguides 508 in the array ofsingle-mode optical waveguides 510 described can simultaneously becoupled to the optical line terminal receiver 128 with nearly unityefficiency.

The design of the single-mode optical waveguides 508 in the array ofsingle-mode optical waveguides 510 and choice of the optical coupler1202 must insure that the signal from any one of the ONU-facing ports122, must arrive at the optical line terminal receiver 128 within atolerance of 40 ps with any other of the ONU-facing optical ports 122.The delay through any of the single-mode optical waveguides 508 in thearray of single-mode optical waveguides 510 must also consider any modaldispersion in the optical coupler 1202. The optical coupler 1202,between the array of single-mode optical waveguides 510 and the opticalline terminal receiver 128, may include a lens, reflective coupling,refractive coupling, proximity coupling, or the like.

It is understood that while the optical line terminal receiver 128 isshown in two segments being coupled to independent instances of theoptical coupler 1202, there may be a convergence of the electronicportion of the optical line terminal receiver 128 that is not shown. Inan alternative construction the optical line terminal receiver 128 mayhave two ports that couple to each of the instances of the opticalcoupler 1202.

The planar lightwave circuit 1302 is an integrated device, which can beused to split the downstream optical signal from the optical lineterminal transmitter 410, in an implementation of a GE-PON or GPON OLT,into the ONU-facing optical ports 122, while simultaneously combiningthe optical power from the ONU-facing optical ports 122 and guiding theupstream signal to the optical line terminal receiver 128 or the secondavalanche photo diode 1306 in a form useful for upstream communicationsat the bit rate conforming to the passive optical network standards.

The above described configuration may allow the use of existingintegrated electronics to perform operations on the upstream signal.Such a configuration might enable a faster manufacturing response timeto a new product requirement. The additional flexibility provided by thepresent invention enhances the manufacturability and design margin forproducts that implement it. For example, the number of ONU-facingoptical ports 122 that can be coupled into a single photo-detector is afunction is inversely related to the size of the photo-detector. Ingeneral, higher-speed (or bandwidth) photo-detectors have smaller activedetection areas.

In the event that the number of desired downstream ports exceeds thenumber that can be effectively coupled into a single photo-receiver onemay desire to use another photo-receiver in order to maintainmanufacturing margin and delivery schedule.

Referring now to FIG. 14, therein is shown a schematic diagram of anoptical line terminal transceiver 1400 utilizing a 2×8 single-modeoptical splitter/combiner in a seventh embodiment of the presentinvention. The schematic diagram of the optical line terminaltransceiver 1400 depicts a planar lightwave circuit 1402 having the 2×2single-mode optical couplers 202 arranged to provide the 2×8 single-modeoptical splitter/combiner 1400 includes a split transmitter path as anexample of the present invention.

The seventh embodiment integrates this invention into the structure ofthe planar lightwave circuit 1402. The eight-port version integratedonto the planar lightwave circuit 1402 utilizes the 2×2 single-modeoptical couplers 202 that are designed to evenly split power from theoptical line terminal transmitter 410 at the wavelength, λ_((Tx)). Inthis example, 1480 nm≦λ_((Tx))≦1500 nm as is required for GigabitPassive Optical Network (GPON) and Gigabit Ethernet Passive OpticalNetwork (GE-PON) systems. The wavelength division multiplexer 404preferentially directs some of the upstream signal at 1260nm≦λ_(Rx)≦1360 nm to the avalanche photo diode 504, without impactingthe flow of the downstream signal through the wavelength divisionmultiplexer 404.

The planar lightwave circuit 1402 structure is designed such that anypath from any one of the eight ports in the array of the ONU-facingoptical ports 122 to the active surface of the avalanche photo diode504, travelling through the optical coupler 1202, are equal within atolerance of 40 ps, which is suitable to enable “on-offkeying”—“non-return to zero” (OOK-NRZ) signals at 1.25 Gb/s as are usedin the upstream direction of a GE-PON or GPON.

The implementation must take into account not only the path lengths ofthe waveguides on the planar lightwave circuit 1402, but also the modaldispersion of the optical coupler 1202, in determining the optimaldesign.

In the seventh embodiment of the invention, the planar lightwave circuit1402 may have the single-mode optical waveguides 508 with the numericalaperture (NA) of 0.22, a cross-section of 4.5-μm square, and the modefield diameter of approximately 3.7 μm at 1310 nm. The single-modeoptical waveguides 508 exit the planar lightwave circuit 1402 in alinear array with 8 μm spacing. Those having ordinary skill in the artwill realize that all eight of the single-mode optical waveguides 508described can simultaneously be coupled to the avalanche photo diode 504with a 65-μm active area by using a lens or proximity coupling at anefficiency approaching unity.

The design of the single-mode optical waveguides 508 and choice of theoptical coupler 1202 must insure that the signal from any one of theONU-facing optical ports 122, must arrive at the avalanche photo diode504 within a tolerance of 40 ps with respect to any other optical pathfrom the same one of the ONU-facing optical ports 122. The total delaymust also consider any modal dispersion in the optical coupler 1202.

The planar lightwave circuit 1402 is an integrated device, which can beused to split the downstream optical signal from the optical lineterminal transmitter 410 and the second OLT transmitter 1404, eachtransmitting an identical downstream signal, in an implementation of aGE-PON or GPON OLT, into the array of the ONU-facing optical ports 122,while simultaneously combining the optical power from the array of theONU-facing optical ports 122 and guiding the upstream signal to theavalanche photo diode 504 in a form useful for upstream communicationsat the bit rate conforming to the passive optical network standards.

The above described configuration may allow the use of existingintegrated electronics to perform operations on the downstream signal.Such a configuration may enable use of multiple lower-cost OLTtransmitters, when optical amplification in the 1480-1500 nm wavelengthrange is not an option. Optical amplification of such signals ischallenged since they require the existence of an S-band opticalamplifier. Semi-conductor optical amplifiers are presently availablewith saturated output powers up to about 13 dBm, equivalent to perhapsfour uncooled directly modulated DFB lasers in parallel in the opticalline terminal transmitter 410. S-band erbium-doped fiber amplifiersbased on fiber with exceptionally high bend-loss above 1530 nm, havebeen demonstrated but are not commercially available Such aconfiguration might enable a faster manufacturing response time to a newproduct requirement. The additional flexibility provided by the presentinvention enhances the manufacturability and design margin for productsthat implement it.

Depending on requirements, the invention can be configured as anintegrated device with two optical splitter and one multipath powercombiners. This can be generalized to different number combinations.

Referring now to FIG. 15, therein is shown a functional block diagram ofa 32-port 10-Gb/s PON OLT transceiver 1500 in an eighth embodiment ofthe present invention. The functional block diagram of the 32-port10-Gb/s PON OLT transceiver 1500 depicts a planar lightwave circuit 1502having the 2×2 single-mode optical couplers 202 arranged to provide the1×32 single-mode optical splitter/combiner 1502 as a further example ofthe present invention.

The eighth embodiment integrates this invention into the structure ofthe planar lightwave circuit 1502. The 32-port version integrated ontothe planar lightwave circuit 1502 utilizes the 2×2 single-mode opticalcouplers 202 that are designed to evenly split power from the OLTtransmitter 1504 at the wavelength, λ_((Tx)). In this case, in the1575-1580 nm wavelength window as is defined by both ITU-T and IEEE foruse in a 10-Gb/s OLT transmitter.

In the near term, amplification of downstream transmitter forgigabit-speed PONs will be challenged. Next generation PONs, defined byboth ITU-T and IEEE have selected a 10 Gb/s OLT transmitter operating inthe 1575-1580 nm wavelength window. L-band EDFAs, are well known in theart. A typical L-band EDFA operates over the 1565-1600 nm window with aflat gain response.

Advantageously, for this application an L-band EDFA 1506 need onlyoperate at a single wavelength between 1575 and 1580 nm A +20 dBconstant output power from the L-band EDFA 1506, is well within thebounds of present technology.

The invention configured to facilitate a 32-port 10G-PON OLT requiresthe L-band EDFA 1506, which amplifies the transmitter signal to aconstant output power of +20 dB, leading to an effective optical powerat each port in excess of +3 dB. The harvested-light 514 from 31 of thesingle-mode optical waveguides 508 are guided to one or morephoto-receivers according to the path length restrictions defined in theinvention. In the implementation above, the multi-mode optical waveguide506 having a ˜200-μm core diameter guides the harvested-light 514 to a200-μm diameter active area version of the avalanche photo diode 504.

Based on the standard performance of standard 1×32 planar lightwavecircuit-based optical splitters a 17 dB loss for the downstream, theL-band EDFA 1506 providing +20 dB should give an effective launch powerfrom each port of +3 dB. On the downstream, even a high noise figure(NF=10 dB) L-band EDFA would contribute a relative intensity noise (RIN)of −146 dB/Hz to the overall downstream signal. This RIN contributes anegligible penalty on the overall 10 Gb/s downstream link budget.

We can expect that the in the upstream direction, there is 2 dB of loss,meaning that a single instance of the optical line terminal receiver 128with a sensitivity of −28 dB will translate to an effective sensitivityof −26 dB at each of the thirty-two ports.

The IEEE specification defines a 1.25 Gb/s upstream at a wavelength inthe 1270-1290 nm range. The ITU-T specification 2.5 Gb/s upstream alsoat a wavelength in the 1270-1290 nm range.

On the upstream thirty-one modes can be coupled to a single multi-modeoptical waveguide with high efficiency. The overall waveguide design iscreated such that from any one of the ONU-facing optical ports 122 allpaths to the optical line terminal receiver 128 are equivalent to withina tolerance of 20 ps. If the NA of a single-mode optical waveguide is0.22, and the waveguide cross-section is a 4.5 μm square, the mode fielddiameter is 3.9 μm at 1270 nm

Thirty-one of the single-mode optical waveguides 508 can be coupled withlow-loss into the optical line terminal receiver 128, which may includethe avalanche photo diode 504, of FIG. 5, having a 200-μm active areadiameter. Alternatively, the thirty one of the single-mode opticalwaveguides 508 can coupled into a single multi-mode slab waveguide (notshown) with dimensions of −200 μm×4.5 μm within planar lightwave circuit1502. Either the array of thirty one of the single-mode opticalwaveguides 508 or the single multi-mode slab waveguide can be coupled tothe optical line terminal receiver 128 for activating the avalanchephoto diode 504 having a 200-μm active area diameter with conventionaloptics or a 0.4 NA multi-mode optical waveguide having a 200-μm activearea diameter APDs with a 0.8 GHz bandwidth are commercially available.

For both the 1.25 Gb/s upstream as used in the IEEE approach, and the2.5 Gb/s upstream defined by ITU-T, 0.8 GHz bandwidth is insufficient tosupport burst-mode reception at 2.5 Gb/s. Since a single wellcharacterized receiver is used, one can expect that an electronicequalization circuit (not shown) may be included in the optical lineterminal receiver 128 to compensate for the limited bandwidth—perhapsnot perfectly, but perhaps enough to justify the ˜15 dB improvement inlink budget.

The design of the single-mode optical waveguides 508 and choice of themulti-mode optical waveguide 506, including the length of the multi-modeoptical waveguide 506, must insure that the signal from any one of theONU-facing optical ports 122, must arrive at the avalanche photo diode504 within a tolerance of 20 ps relative to any alternate optical pathfrom the same ONU-facing optical port 122. The delay through any of thesingle-mode optical waveguides 508 must also consider any modaldispersion in the multi-mode optical waveguide 506.

Referring now to FIG. 16, therein is shown a functional block diagram ofa 32-port 10-Gb/s PON OLT reach extension system 1600 in a ninthembodiment of the present invention. The functional block diagram of the32-port 10-Gb/s PON OLT reach extension system 1600 depicts a 32 port10-Gb/s PON OLT reach extension board 1602 having the 1×32 single-modeoptical splitter/combiner 1502, such as a planar lightwave circuitformed of the 2×2 single-mode optical couplers 202, of FIG. 2, as afurther example of the present invention.

The ninth embodiment integrates this invention into the structure of the32 port 10-Gb/s PON OLT reach extension board 1602 utilizes the 1×32single-mode optical splitter/combiner 1502 for port access to theONU-facing optical ports 122. An ONU transmitter 1610 operates at thewavelength, λ_((Rx)), which in this case, in the 1270-1290 nm wavelengthwindow as is defined by both ITU-T and IEEE for use in a 10G-PON ONUtransmitters. The L-band EDFA 1506 need only operate at a singlewavelength between 1575 and 1580 nm

The downstream signal may be regenerated optically by use of an EDFA,semiconductor optical amplifier, or outside-the-box Raman amplification.In alternate embodiments the downstream signal can be regenerated by anoptical-to-electronic-to-optical (OEO) process. In such a process, anoptical receiver directed toward the optical line terminal, detects thedownstream optical signal generating an electrical signal which drivesone or more downstream transmitters that are coupled into the OLT-facingdownstream port or ports.

A +20 dB constant output power version of the L-band EDFA 1506, is wellwithin the bounds of present technology. The L-band EDFA 1506 willsufficiently drive the down stream port of the 1×32 single-mode opticalsplitter/combiner 1502.

The multi-mode optical waveguide 506 having a ˜200-μm core diameterguides the harvested-light 514 to a 200-μm diameter version of theavalanche photo diode 504. A high speed analog to digital converter 1606may interpret the output of a trans-impedance amplifier 1605 coupled tothe avalanche photo diode 504. The high speed analog to digitalconverter 1606 may provide a stream of digital bits as input to a fieldprogrammable gate array 1608.

A clock may be derived locally or intercepted from the downstreamsignal. All burst-mode circuit functionality, and additionalequalization (e.g. for insufficient receiver bandwidth), even some levelof forward error correction (FEC) can be accomplished digitally withinthe field programmable gate array 1608. In addition digital processingwithin the field programmable gate array 1608 may allow an ONUtransmitter 1610 to send signals back to the central-office basedoptical line terminal (not shown) at constant amplitude, relaxingdynamic range specifications of the optical line terminal burst-modereceiver (not shown).

The embodiment above could accommodate a change in signal format byadjusting the signal processing performed by the field programmable gatearray 1608. The embodiment illustrated above can, for example,accommodate 2.5 Gb/s binary and 5 Gb/s duobinary upstream signalingwithout any hardware modification.

A single mode fiber connector may be coupled to an OLT-facing opticalport 1612 positioned on an edge of the module, and 32 single-mode fiberconnectors are located on the ONU-facing side. The number of ports onthe ONU-facing side may be adjusted as required by the application.Within the module a three port WDM coupler 1604 can be used to directthe downstream signal to the optical amplifier or optical receiver (inthe case of OEO downstream regeneration). The implementer may choose toinclude an ONU within the module to communicate module status andtelemetry information to the optical line terminal.

One having ordinary skill in the art of PONs, will realize that themodule will provide the most utility to the PON network operator if itcan operate transparently, and autonomously from the OLT and ONUterminal equipment. Earlier attempts at regenerating GPON upstreamsignals using an OEO approach, have demonstrated a reduced dynamicrange, in part due to the fact that an OLT-located receiver has accessto a reset-signal, which is fed to the receiver as a direct electricconnection from the OLT MAC (media access control.) Operation of a PONextension system, independent, autonomously, and remotely from the OLTprecludes access to the reset signal.

One solution to achieve high-performance independent receiver operationis to utilize a better receiver decision-thresholding anddecision-making system compared to the (analog) circuit standard in mostOLT burst-mode receivers. One approach, to improve and make more robustthe signal reception and discrimination is to sample the upstreamreceiver signal or signals, and utilize digital signal processing, toadd a level of pre-processing or equalization to the signals, and toutilize more flexible and robust decision algorithms than possible in an(analog) electronic circuit.

Advantageously, this processing can also compensate for any non-idealityin the receiver such as penalty from utilizing a larger active areaphoto-receiver (to capture more upstream light), at the expense ofelectrical detection bandwidth or modal dispersion from the multi-modeoptical waveguide used in some embodiments.

Referring now to FIG. 17, therein is shown a functional block diagram ofa 32-port 10-Gb/s PON OLT reach extension system 1700 in a tenthembodiment of the present invention. The functional block diagram of the32-port 10-Gb/s PON OLT reach extension system 1700 depicts an externalversion of the 1×32 single-mode optical splitter/combiner 1502 coupledto redundant versions of the board 1702, such as a pair of the 32-port10-Gb/s PON OLT reach extension systems 1600, of FIG. 16.

A first optical regenerator 1702 may include the three-port WDM coupler1604, the L-band EDFA 1506, the multi-mode optical waveguide 506, theavalanche photo diode 504, the trans-impedance amplifier 1605, the highspeed analog to digital converter 1606, the field programmable gatearray 1608, and the optical transmitter unit 1610. The three-port WDMcoupler 1604 may provide the source for the OLT-facing optical port1612.

A second optical regenerator 1704 is identically configured to have thesame functional blocks as the first optical regenerator 1702. The L-bandEDFA 1506 may be coupled through a single mode fiber to a single modeswitch 1706. An identical connection is made between the second opticalregenerator 1704 and the single mode switch 1706. An output of thesingle mode switch is the downstream port of the 1×32 single-modeoptical splitter/combiner 1502.

The first optical regenerator 1702 and the second optical regenerator1704 comprise a redundant back-up electronic system capable of extendingthe field serviceability of the 32-port 10-Gb/s PON OLT reach extensionsystem 1700. Due to the completely passive nature of the 1×32single-mode optical splitter/combiner 1502, its field longevity can beenhanced by having the redundant electronic sets provided by the firstoptical regenerator 1702 and the second optical regenerator 1704.

A multi-mode connection is made between the upstream port of the 1×32single-mode optical splitter/combiner 1502 and a multi-mode switch 1708.The outputs of the multi-mode switch 1708 are coupled to the multi-modeoptical waveguide 506 of the first optical regenerator 1702 and thesecond optical regenerator 1704.

With such high N, some redundancy may be desirable. In general, even themost reliable active electronics and electro-optics have much lowerreliability compared to passive optical elements. The illustration aboveillustrates how (active) equipment redundancy and path (to the OLT)redundancy may be accomplished.

Referring now to FIG. 18, therein is shown a functional block diagram ofa hybrid-fiber coax optical network repeater 1800 in an eleventhembodiment of the present invention. The functional block diagram of thehybrid-fiber coax optical network repeater 1800 depicts a planarlightwave circuit 1802 having eight of the ONU-facing optical ports 122and implementing a 1×8 single-mode optical splitter/combiner by couplingthe 2×2 single-mode optical couplers 202 by the single-mode opticalwaveguides 508.

The planar lightwave circuit 1802 may be mounted on a carrier board 1804with the down stream port of the planar lightwave circuit 1802 coupledto a transmitter port 1806 of the carrier board 1804. A hybrid-fibercoax networks (HFC) return path receiver 1808 is coupled to thesingle-mode optical waveguides 508 of the planar lightwave circuit 1802via multi-mode optical waveguide 506. The RF amplifier of the HFC returnpath receiver 1808 is coupled to the upstream RF output 1810 of thecarrier board 1804. The single-mode forward-path optical input port1806, will connect to a forward-path transmitter (not shown) or anoptical amplifier (not shown).

The diagram of FIG. 18 shows an example of an alternate implementationof the invention, which combines the function of several of the modulesinto a single module with better performance, significantly smallersize, and wavelength independence. The planar lightwave circuit 1802works as well for a 1310 nm Radio Frequency over Glass (RFoG)return-path signal, as it does with a 1610 nm RFoG return-path signal.

The hybrid-fiber coax optical network repeater 1800 is an integratedforward-path optical splitter/power-combining return-path receivermodule for use in HFC and RFoG networks. Apart from its multi-modeoptical waveguide input 506, the HFC return path receiver 1808 is wellknown in the prior art. According to the invention the multiple pathsfrom any of the ONU-facing optical ports 122 to the photodiode 1811 mustbe equal within a tolerance of 851 ps. The planar lightwave circuit 1802implementation as illustrated may be implemented with fused-fibercouplers 2×2 single-mode optical couplers 202.

The return-path RF signals are modulated on subcarriers in the f=10-80MHz range according to the DOCSIS 3.0 specification. It is well known inthe prior art that for combining identical RF subcarrier multiplexed(SCM) signals with a time delay of Δt, the carrier-to-noise ratio (CNR)is penalized according to the equation:

CNR=CNR_(max) cos²(πf _(max) Δt)  EQ 5

For a maximum 0.1 dB penalty over all frequencies, Δt<851 ps. In glass(n=1.5) this is equivalent to a length of 17 cm. This length toleranceis easily managed even in splicing fibers. Accordingly, although not ascompact as a planar lightwave circuit implementation, a fully feasibleimplementation can be based on splicing 2×2 50/50 single-modefused-fiber couplers while maintaining a 17 cm length tolerance,according to the invention.

Referring now to FIG. 19, therein is shown a functional block diagram ofan optical line terminal transceiver 1900 utilizing a 1×(N/2) singlemode splitter/combiner, in a twelfth embodiment of the presentinvention. The functional block diagram of the optical line terminaltransceiver 1900 depicts the (N/2) 2×2 single-mode optical couplers 202having the array of the ONU-facing optical ports 122. Additional opticalsplitting of the downstream signal is facilitated by a single-mode1×(N/2) splitter 1902. Each of the harvesting ports 306 on the 2×2single-mode optical couplers 202 is routed to the optical line terminalreceiver 128 for conversion to an independent electrical signal. Sinceeach of the harvesting ports 306 has a unique route tooptical/electrical converter 412, the length of its routing is alsoindependent and does not have a critical timing relationship to otherroutes.

The routing of the first line of the 2×2 single-mode optical couplers202 to the optical line terminal receiver 128 includes ½ of the totalsignal content from the array of the ONU-facing optical ports 122 and isgreater than or equal to the amplitude of all of the remainingunrealized 2×2 ports within the 1×(N/2) single-mode splitter 1901.Routing only the first line of the unused outputs of the 2×2 single-modeoptical couplers 202 will approximate the maximum result within 3 dB andis sufficient to provide reliable and robust communication.

Referring now to FIG. 20, therein is shown a functional block diagram ofa passive optical network optical line terminal line card 2000 utilizingthe planar lightwave circuit 502 in an embodiment of the presentinvention. The functional block diagram of the passive optical networkoptical line terminal line card 2000 depicts the small form-factorpluggable mechanical interface 602 having the electrical interface 606,such as a small form-factor pluggable industry standard interface forpassive optical network support. The electrical interface 606 has anindustry accepted specification for mechanical and electricaltolerances.

The small form-factor pluggable module 608 may couple to the electricalinterface 606 and the mechanical interface 602 through its electricalinterface 604. In an embodiment of the present invention, as an example,the small form-factor pluggable module 608 may include the optical lineterminal transmitter electrical circuit 610 that manages thetransmission and receiving of the data between the small form-factorpluggable module 608 and the optical network units 124, of FIG. 1, thatare distributed along the optical network (not shown). The optical lineterminal transmitter electrical circuit 610 may provide an electricalinterface that drives an optical line terminal bidirectional opticalsub-assembly 2002.

The optical time-domain reflectometer (OTDR) probe 1106 may be coupledto the planar lightwave circuit 502. The presence of the multi-modeoptical waveguide 506 connection from the planar lightwave circuit 502to the optical time-domain reflectometer (OTDR) probe 1106 allows theinclusion of the harvested-light 514 for network monitoring purposes. Byusing the harvesting technique of the present invention, a −16.3 dBimprovement in the signal returned to the optical time-domainreflectometer (OTDR) probe 1106 can be achieved. This significantlyimproves the sensitivity and accuracy of the readings taken across thesingle-mode optical fiber 126, of FIG. 1. Advantageously, the opticalisolation between the bidirectional optical sub-assembly 2002 and theOTDR probe 1106 is also improved.

Having the optical time-domain reflectometer (OTDR) probe 1106 embeddedwithin the small form-factor pluggable module 608 may provide areal-time analysis capability for determining the condition of thesingle-mode optical fiber 126 that is coupled between the opticalnetwork units (ONU) 124, of FIG. 1, and the ONU-facing optical ports 122of the small form-factor pluggable module 608. The optical time-domainreflectometer (OTDR) probe 1106 may be used for estimating the length ofthe single-mode optical fiber 126 and overall attenuation, includingsplice and mated-connector losses. It may also be used to locate faults,such as breaks, and to measure optical return loss.

It will be understood by those skilled in the art that the invention canbe essential to the daily maintenance and support of the single-modeoptical fiber 126, of FIG. 1, used in many types of point-to-multi-pointoptical communications networks, including but not limited those definedby the 10G-EPON standard, the emerging ITU-T 10G-PON standard, RF overglass (RFoG) networks, other hybrid-fiber coax networks, and otherpoint-to-multi-point optical (or partially optical) networks.

Referring now to FIG. 21, therein is shown a flow chart of a method 2100of operation of an optical network communication system in a furtherembodiment of the present invention. The method 2100 includes: providinga planar lightwave circuit including: connecting 2×2 single-mode opticalcouplers in an array for forming a 1×N single-mode opticalsplitter/combiner, and routing harvesting ports to a receiver forcollecting harvested-light, from two or more of the harvesting ports, inthe receiver wherein one of more of the harvesting ports is from the 2×2single-mode optical couplers in a block 2102; transmitting to opticalnetwork units through the planar lightwave circuit at a first wavelengthin a block 2104; and interpreting a response from the optical networkunits at a second wavelength through the harvested-light in a block2106.

The resulting method, process, apparatus, device, product, and/or systemis straightforward, cost-effective, uncomplicated, highly versatile,accurate, sensitive, and effective, and can be implemented by adaptingknown components for ready, efficient, and economical manufacturing,application, and utilization.

Another important aspect of the present invention is that it valuablysupports and services the historical trend of reducing costs,simplifying systems, and increasing performance. These and othervaluable aspects of the present invention consequently further the stateof the technology to at least the next level.

While the invention has been described in conjunction with a specificbest mode, it is to be understood that many alternatives, modifications,and variations will be apparent to those skilled in the art in light ofthe aforegoing description. Accordingly, it is intended to embrace allsuch alternatives, modifications, and variations that fall within thescope of the included claims. All matters hithertofore set forth hereinor shown in the accompanying drawings are to be interpreted in anillustrative and non-limiting sense.

What is claimed is:
 1. A planar lightwave circuit comprising: 2×2 single-mode optical couplers coupled in an array to form a 1×N single-mode optical splitter/combiner with two or more harvesting ports from uncommitted ports of the 2×2 single-mode optical couplers, where N is greater than 2, each of two or more of the 2×2 single-mode optical couplers includes a port uncommitted and available to become one of the harvesting ports; and a waveguide configured to form a single composite harvesting port efficiently routing a combined harvested optical signal. 